This invention relates to a liquid crystal display combined with a signal input tablet which includes an optical system for parallax compensation and which will be suitable for combining a flat surface display such as a liquid crystal display with an electromagnetic signal input tablet or an ultrasonic signal input tablet.
Conventional liquid crystal displays combined with a signal input tablet have the construction such as shown in FIG. 2. A liquid crystal tablet 2 is disposed on an electromagnetic signal input tablet 1 and data is written by an electromagnetic pen 3 while viewing the displayed characters. It is a customary practice to cover the liquid crystal sheet 2 with protective glass 4 lest it be damaged by writing pressure when the electromagnetic pen 3 rubs the display surface. Particularly when the liquid crystal display is used for a work station or the like, its size is as great as about an A5 size, and thick protective glass 4 must be used lest glass be bent by the writing pressure. In this case, if the character displayed at a point A is viewed, the character is recognized by eyes as if it were displayed at a position B which is somewhat closer to the eyes due to refraction of the glass of the liquid crystal plate and protective glass 4 because the positions of the eyes are slantingly upward at an angle of about 45.degree. . Therefore, if the electromagnetic pen 3 is put to the position B, a position C on the electromagnetic signal input tablet 1 is pointed and a pointing position error corresponding to parallax d develops.
If this error is always constant, this problem can be solved by deviating in advance the electromagnetic signal input tablet 1 by the parallax d. However, the error cannot be corrected accurately because the height of the eyes changes or because the parallax d differs between the central portion and the peripheral portion. In other words, assuming that the height of eyes is h, the horizontal distance from the eyes to the pen tip is l, the thickness from the liquid crystal display 2 to the surface of the protective glass 4 is t and the index of refraction of glass is 1.5, then the parallax d is given by the following formula: ##EQU1## where .theta.' is an angle of ray of light outgoing from the display surface to the normal (see FIG. 2). The diagram showing the change of the parallax d when l is changed is put on "No. 31 Information Processing Society of Japan, Report 1N-4". When h=300 mm and l=250 mm, for example, d.apprxeq.0.9 mm and the error becomes extremely great because resolution of the electromagnetic signal input tablet 1 is 10 line/mm.
The prior art technique described above has no means for compensating for the parallax d and cannot easily and completely pointing position error.